One lot, two duplex's, counted as 4 units. Is it "rented every single day" if any of the units were empty, or do all 4 have to be rented for the property as a whole to be considered "rented every single day"?
You have to treat each unit separately. You can't have some units treated one way and others treated differently and then claim it as one unit.
Sorry, that's still not making sense. Since there is an option for a property to be "multi-family", that implies that the building is treated as a single entity for tax purposes. Unless you're supposed to list each individual unit on its own? Then divide improvement costs to the entire building by the number of units, like if you put a new roof on a duplex? Are corporations that own buildings with hundreds of apartments filling out individual forms for each individual apartment or the building as a whole?
To answer the first question. They do not all have to be rented. It is still a rental property as long as it was available for rent during the entire year (the fact that it wasn't rented will not make it a personal use second home). You will just put "0" for the question on "the days rented at FMV" if only available to rent but not rented.
The confusion that is occurring is are these "units" a multifamily dwelling, or are they considered condos so they stand as individual units?
If this is a multifamily dwelling you can include common expenses together.
If these are actually single units, you will have to allocate the cost.